Nest 3rd generation Thermostat and C-Plan system

[abel88]

Hi all,

I'm looking to get the latest Nest Thermostat (3rd Gen) and have a c-plan HW gravity feed hot water system wired like the following diagram (although I have only to wires from the Room Stat):

My existing programmer is wickes/siemens RB9_i, which doesn't allow for HW and Heating to be on independantly (HW has to be on to use central heating)

My question is will the Nest allow me to control HW and CH seperately and also how will I wire the 2 wires going to the existing programmer? Will it be wire going to term 3 (HWON) on the RB9 to terminal 6 on the Nest and wire on term 4 to terminal 3 on the Nest (see below Nest installation diagram)?

Many thanks for your help.

 

[hertsboilers]

Nest 3G is not compatible with C-plan.

 

[abel88]

So, 2nd Gen only... Of the top of your head, would Hive be compatible? Is it possible to have independant control of HW and CH on a C-plan system?

Thanks

 

[hertsboilers]

Nest 3G is not compatible with C-plan.

Sorry, but need to read the manual before further confirmation.

 

[hertsboilers]

Okay, a final answer now.

If your existing programmer doesn't allow you turn on your heating alone, then you have a gravity system, but not with a two port valve like the C plan in your first post. In such a case, any Nest, either 2G or 3G, is not compatible with your existing wiring.

 

[hertsboilers]

 

[picasso]

C plan is hot water priority, not great on a gravity hot water system,(holds the heating off while the cylinder reheats) why not upgrade to fully pumped and then you can have any stat you like.

 

[D_Hailsham]

C plan is hot water priority

Yes it is, but I suspect you were thinking of the W Plan which is also HW priority. The difference being that the C plan is for gravity HW/pumped heating while the W plan is for fully pumped.

I can't see why the OP can't use the Nest 3G. The C Plan wiring is such that the CH side will be deactvated (and also the pump) when HW is called. When HW is satisfied the system reverts to CH control.

Incidentally, I assume the OP means a RWB9 programmer. If so, this could have been converted for use with a C Plan by changing Dip Switch 5 (on back of programmer). HW and CH times would then be completely independent. (The reset button (on the front) then has to be pressed with a biro or similar to complete the process.)

 

[abel88]

Hertsboiler, I do have a 2 port valve on the system.

D_Hailsham, yes the dip switches are set for gravity fed hw. Are you saying if i change the dip switches to fully pumped it will work independently of each other?

 

[abel88]

Also, at the moment hw and ch run simultaneously, ch does not turn on without hw being on but ch stays on when hw is called for (at least i assume it is) maybe it's not a c-plan after all?

 

[hertsboilers]

It seems your current wiring is wrong.

Also, at the moment hw and ch run simultaneously, ch does not turn on without hw being on but ch stays on when hw is called for (at least i assume it is) maybe it's not a c-plan after all?

So your current wiring is neither C plan or the plan I put on above #6.

 

[abel88]

Sorry if I'm confusing things but I'm inexperienced in these matters. My set up is exactly as in the c-plan above, room stat, 2 port zone valve, cylinder stat, pump and programmer. The only thing is i can't confirm the exact wiring in the junction box but the wiring to the siemens programmer is as in the diagram. The instructions stated to run the system as a gravity hw system as that is what i have, although maybe it's possible to run it otherwise. I'm just after some help understanding what i have and if a nest stat is suitable or not. I appreciate all your comments and advice.

 

[hertsboilers]

Sorry if I'm confusing things but I'm inexperienced in these matters. My set up is exactly as in the c-plan above, room stat, 2 port zone valve, cylinder stat, pump and programmer. The only thing is i can't confirm the exact wiring in the junction box but the wiring to the siemens programmer is as in the diagram. The instructions stated to run the system as a gravity hw system as that is what i have, although maybe it's possible to run it otherwise. I'm just after some help understanding what i have and if a nest stat is suitable or not. I appreciate all your comments and advice.

If yours is a corrected C-plan, you should be able to control hot water and central heating independently. Now you have either got your current programmer setting wrong (gravity instead of fully pumped), or wiring wrong, as you can't turn only one of them on at any time.

Get a RGI in, or call a sparkie now.

 

[abel88]

Thanks for your help, so if i change the dip switches on the programmer to fully pumped it will work although the water is gravity fed?

 

[shambolic]

No it won't.
Now get someone in to fit your nest/hive before you wreck your wiring or electrocute yerself.